Accelerated ions (atoms or molecules with positive or negative electrostatic charges) find important uses in industry and science. Historically, ions which have been accelerated to high velocities have been used in particle physics and condensed matter physics to probe the fundamental laws of the universe. Over time, practical uses have arisen for accelerated ions. For example an ion accelerator allows the precise injection of ions onto a substrate. In the tool industry, this capability has been used to develop new surface hardening techniques. In the semiconductor industry, ion accelerators have been critical to the implantation of doping ions which create transistors, diodes and gates on a semiconducting substrate, such as silicon.
Ion accelerators, of the tandem type where first negative ions are accelerated then striped to form positive ions, have also found use in such fields as archeology, where the ability to eliminate molecular isobars such as CH.sub.2 has made possible dramatically increased accuracy of carbon-14 dating through a mass determination of a representative sample of carbon atoms. This technique allows the determination of the carbon-14/carbon-12 ratio with a small sample and with much higher precision, because decay of the carbon-14 atom is not necessary to detect its presence.
Ion or particle accelerators use static or electromagnetic fields which interact with the static charge on the ion to produce an accelerating force. Thus, the accelerator requires a source of ionized atoms or molecules to be accelerated.
Ions may readily be extracted from a plasma formed of the molecular species of interest. A plasma can be created by electric arc, but a microwave-heated plasma produces a more durable, more controlled ion source. The plasma is typically of fairly low density, being formed of a gas with a pressure of approximately 0.01 Torr. This low pressure allows a sufficient mean-free path of the formed ions, such that they can be drawn out of the plasma chamber by a suitable static or electromagnetic field and introduced to the particle accelerator.
As gas is withdrawn from the plasma chamber of the ion source, replacement gas must be supplied. This is typically done through a metering valve which allows a very precise, low flow of gas to the plasma chamber, which balances the drain of ions which are extracted.
In steady-state operation, the inflow of gas equals the outflow of ions. For initiation of a plasma, however, a higher density of gas is required. The higher density of gas is required because, in order to be heated by a microwave radiation, the gas must absorb energy from the excitation source. The relative opaqueness of the gas to energy depends on its density. Once ionized, the free electrons in the plasma are extremely opaque, and so a relatively lower pressure of gas is sufficient to sustain the plasma. Thus, in order to initiate the plasma in the ion source, the supply of gas to the ion source must be increased so as to increase the pressure in the ion source approximately a factor of ten over its steady-state pressure.
Pressure is normally increased by opening the metering valve and then immediately stopping it down. While a seemingly straightforward process, in practice, it is tricky and, further, not easily subject to automatic control. In the past, when the acceleration of ions was exclusively the domain of scientists, principally particle physicists, the difficulty of starting the ion source was an accepted part of the research process. However, as accelerators have become more ubiquitous, their users have included scientists from other disciplines, from chemistry to biology to semiconductors, who are less interested in the particularities of the ion source and the accelerator, than in the end use of the ion beam produced therefrom.
Similarly, the industrial user demands a reliable, consistent system for initiating the ion source, which does not require skill and experience on the part of the operator. Because of the complexity of the feedback between gas flow and the ion source, automatic control systems have proven less than satisfactory at solving the problem of ion source plasma initiation.
What is needed is an ion source initiation system for reliably initiating the plasma in the ion source without operator intervention.